Army Package Power Reactor
Water Treatment and Waste Disposal A. LOUIS MEDIN Alco Products, Inc., Schenectady, N. Y.
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APRIL1957, the pressurized water Army Package Power Reactor, at Fort Belvoir, Va., became critical. It is a IO-mw. heat source capable of producing 2000 kw. of electricity, and with the exception of the nuclear heat source, differs from a normal power plant in that its steam generator is made of Type 304 stainless steel instead of carbon steel. In the primary loop, water circulated a t 1200 p.s.i. and 4000 gallons per minute is heated to 450' F. by the fission process. In the steam generator, the primary water temperature is reduced to about 430' F. in the process of transferring its heat. Steam a t 200 p.s.i.a. and 382O F., produced on the secondary side, is superheated an additional 25' F. in the superheater section of the steam generator. For chemical data on the various water systems, numerous sampling points were established. The design and equipment selected for this reactor have proved satisfactory. During the start-up period, of course, normal problems appeared, but not with the reactor core itself. Pump and valves often leaked, but no serious failures occurred. The core is fully enriched uranium with plate-type fuel elements clad in Type 304 stainless steel. The primary system is principally of the same type of steel and contains only minor amounts of construction materials such as stellite and 17-4 steel. I n the primary system, high purity water is maintained to minimize build-up of excessive radioactivity caused by either impurities or corrosion products, and to prevent deposition among close tolerance mechanisms and the fuel elements. Also, high purity water minimizes corrosion resulting from oxygen or carbon dioxide and prevents formation of corrosive acids caused by nuclear radiation. To maintain the primary water at less than 2 p.p.m. of total solids, a bypass purification system is employed which includes oxygen control. A portion of the primary water is removed, purified, and no significant recirculated. Thus, amount of primary water need be discharged to the environment. The blow-down water is removed from the reactor downstream of the steam generator return leg a t a temperature of about 430' F. River water, used in the heat exchanger to reduce the temperature
to less than 120' F., protects the downstream resins from serious thermal damage. A motor-operated throttling valve reduces the pressure from 1200 to less than 100 p.s.i. Should a leak develop in the blow-down heat exchanger and activity enter the river water, a radiation monitor will automatically divert this water to the hot-waste tank. Leaving the vapor container, the water enters the demineralizer room where it is sent to one of two mixed-bed demineraliqers. Should a fuel element rupture and send fission products into the primary water, a radiation monitor will automatically divert this water also to the hot-waste tank. After leaving the demineralizer, the water is passed through a micrometallic filter of porous Type 304 stainless steel, 8 inches high by 4 inches in diameter, with a total filter area of 1 square foot. Should it be necessary to remove the filter, a bypass has been provided. The water is collected in a 5000-gallon Type 304 stainless steel holdup tank and sent back to the primary system by means of one of two positive displacement pumps. This holdup tank has a center annulus to provide about an 8-hour holdup of primary blow-down water. Thus, short half-life radioactive nuclides will decay without radiation hazard to personnel. Most of the valves and spare connections are located in a pipe trench within the demineralizer room so that water can
be easily diverted in many different patterns-e.g., if necessary, water can be directed to and from such equipment as the demineralizers, hot-waste tank, and filter. T h e mixed-bed resins are a mixture of a sulfonic acid-type cation in the hydrogen form, and a strongly basic anion in the hydroxyl form. These were especially treated by the manufacturer to remove any soluble organic contaminants present. This guarantees a minimum leakage of water soluble organic contaminant materials from the resins. As received, the resins are in the regenerated hydrogen and hydroxyl form and have a total minimum capacity of 12 kilograins per cubic foot, calculated as calcium carbonate. Since the resins will remove radioactive constitutents also, each demineralizer is shielded behind 2 feet of concrete designed to permit maintenance or replacement of one demineralizer while the other operates. During initial start-up, the purification rate, which depends primarily on the corrosion rate, is usually higher because the corrosion rate has not been stabilized. After stabilization, the corrosion rate is estimated to average approximately 0.05 mg. per sq. cm. per month. T o reduce corrosion, oxygen which enters the primary system either by unintentional air leakage or from dissociation of water which occurs under radiation flux, is held to a minimum.
The reactor is a 10-mw. heat source capable of producing 2000 kw. of electricity VOL. 50, NO. 7
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JULY 1958
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FEEDWATER
GENERATOR
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tially passing through a preheater deaerator. Overheads of the evaporator generally average greater than 300,000 ohm-cm. Chlorides were not detectable in normal operation. Waste Disposal
REACTOR
THROTTLING VALVE
TO WASTE TANK DEMINERALIZERS
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Maintaining high purity water in a primary system requires a bypass purification using mixed-bed demineralizers
Before the reactor became critical, oxygen was scavenged with hydrazine fed in concentrations of 0.04 to 1% to the suction side of the primary make-up pumps. This method was successful and prevented solids build-up. Oxygen concentration dropped from 5.4 to 0.01 p.p.m. rapidly, while the average system temperature was 325" F. The over-all performance of the mixedbed resin in the demineralizer has been satisfactory. Because this unit must remove both suspended and dissolved solids from the blow-down stream, an efficiency reduction was expected; but this has not been serious or deleterious. Conductivity and pH remained relatively constant throughout a 700-hour high performance test and specific resistance of the coolant was usually over 600,000 ohmcm. Resistance of the demineralizer eflluent was about 2 to 4 X lo6 ohm-cm. Although the demineralizer effluent pH was generally less than 7.0, the primary water itself varied between 7.2 and 9.0. The alkaline pH was believed caused by ammonia which, in the presence of hydrogen under a radiation flux, is synthesized from air that leaks into the primary make-up system. Hydroxyl ions do not contribute to the radioactivity of the primary coolant, and experiments show that a higher pH is beneficial in corrosion control; therefore, the system is considered to be operating satisfactorily. Under a radiation field, either hydrogen will react with oxygen to form water, or water will break down to form oxygen plus hydrogen. An excess of hydrogen will shift the equilibrium to prevent decomposition of water, and amounts exceeding 5 to 10 cc. per liter will eliminate all dissolved oxygen. Hydrogen fed into the bottom of the 5000-gallon makeup tank was maintained at a pressure of about 35 p.s.i.a. above the water level. Hydrogen concentrations of 20 to 40 cc. per liter of water have been main-
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tained consistently since the reactor has been in operation, and no significant oxygen has been detected in the primary loop since the reactor became critical. Secondary System
With a stainless steel generator, chloride and oxygen control become extremely important because of stress corrosion. Generally chloride stress corrosion depends on four factors-chloride content, oxygen content, metal stress, and crevices. Although chloride concentration in the boiler is low, it may, through evaporation, concentrate by a factor of from 10 to 100 in crevices such as at the tube-to-tube sheet junction. If the metal is under heavy stress, then stress corrosion can result. Most investigators agree that chloride stress corrosion cannot occur without oxygen, but, threshold amounts of either oxygen or chlorides have not been well defined. Therefore, oxygen and chloride concentration was established at 0.03 and 0.5 p.p.m., respectively. These limits \vere thought severe but practical. The results have been good. Concentration for chloride averages about 0.2 to 0.3 p.p.m. and oxygen seldom exceeds 0.03 p.p.m. Oxygen control on the secondary side is maintained by a combination of air ejectors at the condenser hot well and a chemical scavenger, a sodium sulfite solution fed to the discharge side of the boiler feed pump. Because all the steam is condensed and returned to the steam generator, very little make-up water is required for the secondary system. Generally it is used only to replenish blow-down sample losses and unaccountable leakage. A flex-tube evaporator makes distilled water. Service water, having a hardness of 45 p.p.m, as calcium carbonate and 85 p.p.m. total dissolved solids, of which approximately 10% is chlorides, is admitted to the evaporator after ini-
INDUSTRIAL AND ENGINEERING CHEMISTRY
Because the reactor site is relatively small, the cost of waste disposal must be held to a minimum and yet guarantee absolute safety. Under normal operating conditions, demineralizer resins which have collected activity from the primary blow-down and small amounts of laboratory sampling wastes can be the only source of radioactive wastes. Because the design was intended to prevent continuous discharge of primary coolant water to the river, expensive evaporators, storage tanks, and auxiliary features have not been needed. The principal item for waste storage, a 5000-gallon underground carbon steel tank, is designed to collect primary water should a large build-up of activity occur from a ruptured fuel element. This tank, designed to store waste at medium temperatures and low pressure, is cradlemounted in an underground leakproof concrete vault and vented to the stack. The vapor discharge can be regulated by a manually controlled valve after the acrivity level and discharge rate have been established. Two small pumps in series discharge the waste either directly to the river or through the demineralizers if activity is too high. Two 250-gallon laboratory waste tanks lined with Amerplate, one of which can be emptied while the other is used, receive laboratory and sampling wastes. The primary purification demineralizers have been sized to operate effectively for 3 to 6 months. O n exhausrion, the resins will not be regenerated. The volume of radioactive wastes now contained is small, and disposal of larger volumes of regenerating chemicals and water is more complicated. An economical unit was designed and purchased for disposal of both resins and container after exhaustion. The demineralizer is equipped with fittings which permit rapid removal and assembly. Consequently, the demineralizers will be removed from the system when exhausted, placed in a lead cask, and shipped to a burial ground.
Conclusions Maintaining high purity Ivater in a primary system requires a bypass purification system using mixed-bed demineralizers. Type 304 stainless steel requires absolute minimum oxygen and chloride concentrations in the secondary system. Waste disposal systems and equipment are reduced by using a closed primary loop. RECEIVED for review September 7, 1957 ACCEPTED January 15, 1958
